1. Field of the Invention
The invention relates to a method for the determination of the spatial and of the spectral distribution of the nuclear magnetization in a region under investigation, in which, in the presence of a homogenous steady magnetic field, a number of magnetic field sequences act on the region under investigation, each sequence comprising at least one magnetic high-frequency pulse for the excitation of nuclear magnetic resonance and subsequently a plurality of periods of a magnetic gradient field with a gradient periodically varying its polarity, after which the echo signals generated in this procedure in the region under investigation are converted into digital samples and these are subjected to a discrete Fourier transformation.
2. Description of the Prior Art
Such a method is known (Matsui et al, J.Am.Chem.Soc. 1985, 107, pages 2817 to 2818). Furthermore, the invention relates to an arrangement for carrying out the method.
In this procedure, each sequence comprises two high-frequency pulses, which flip the magnetization vector in the region under investigation by 90.degree. and by 180.degree. respectively from its previous position. Between the two pulses or directly thereafter, for the purpose of the phase coding a gradient field is actuated, the gradient of which is varied from sequence to sequence. After this gradient field has been switched off again, there is a further magnetic gradient field, the gradient of which extends in a direction perpendicular to the (y-)direction of the gradient of the previously mentioned gradient field (i.e. in the x-direction) and varies periodically from a positive to a negative value. After each reversal of the gradient, an echo signal is generated. The echo signals which are generated in the case of positive (or in the case of negative) gradients are converted into digital samples and subjected to a Fourier transformation. The digital samples created in this manner may be classified in each instance in three different groups: The first group comprises the equidistant samples of an echo signal. The second group comprises samples which occur in varying sequences, in each instance in the case of the same echo signal and in the same phase in relation to the periodic gradient field, and the third group comprises the samples which occur in different echo signals of the same sequence, in each instance in the same phase with respect to the gradient field. The temporal interval of the samples of a group is in each instance the same; it is smallest in the first group and greatest in the second group.
The groups, thus obtained, of samples are subjected to a three-dimensional discrete Fourier transformation, which gives the nuclear magnetization in the region under investigation as a function of the position (x, y) and as a function of the frequency (more precisely: as a function of the frequency difference from the central frequency of the high-frequency excitation pulse.
In this procedure, the bandwidth, within which the nuclear magnetization can be recorded, corresponds to the frequency of the magnetic gradient field. If the latter has, for example, a period of 4.992 ms, then this results in a bandwidth of 200.3 Hz. This frequency range can be resolved into a number of subsidiary ranges, the number of which corresponds to the number of periods of the periodic gradient field within a sequence.
In the known method, the periodic gradient field exhibits a practically quandrangular temporal progression, so that the samples are always recorded at points in time at which the gradient is constant and either positive or negative (in reality, the temporal progression is not precisely quandrangular, and indeed the reversal time (50 .mu.s) is shorter than the temporal interval between two samples (78 .mu.s), so that the echo signal is always sampled at points in time at which the gradient is constant and either positive or negative).
The spatial resolution in the direction in which the gradient of the periodic magnetic gradient field extends is restricted; the maximum spatial frequency in this direction is proportional to the temporal integral over the positive or the negative part of the period of the gradient. An increase in this value by increasing the gradient is not possible if the current has already reached its maximum value through the gradient coils by means of which the gradient field is generated. Furthermore, the increasing of the resolution by increasing the period of the periodic gradient field is for practical purposes not possible, because this would result in a decrease in the resolvable bandwith which corresponds to the reciprocal of the period.
In the case of nuclear spin investigation devices for the investigation of the human body, the spatial resolution is, in practice, even more severely restricted. In this connection, the gradient coils must indeed be so large that there is room for a patient between them. As a result of this, they possess a greater inductivity and store more electromagnetic energy than smaller coils. As a result of this, the reversal phase from a positive to a negative gradient (and vice versa) lasts for a substantially greater length of time than in the known method, so that the duration of the reversal phase can amount to a multiple of the temporal interval between two successive samples of an echo signal generated in the region under investigation. In this connection, the sampling of the echo signals must be restricted to the time intervals in which the gradient is constant. However, the temporal integral over the gradient during this interval is (in the case of equal intensity of the gradient and in the case of equal period) even smaller than in the case of a quadrangular progression of the magnetic gradient field, resulting in a reduced resolution.
The object of the present invention is to provide a method with which, on the other hand, in the case of a progression--temporarily differing from the quadrangular form--of the periodic gradient field an improved resolution is obtained, as well as an arrangement for carrying out this method.